1. Field of the Invention
The present invention relates to an illumination device for illuminating a document and an image scanning device (image sensor) in which the illumination device is incorporated.
2. Description of the Prior Art
A facsimile machine, a copying machine, a scanning device and the like are provided with an illumination device in which a bar-shaped light guide for irradiating a line-shaped light on a document is incorporated, and with an image scanning device provided with a lens array for causing the reflected light beams from the document to focus onto a line image sensor.
As shown in Patent Document 1 and FIG. 12, the illumination device is generally constructed in such a manner that the bar-shaped light guide 101 is fitted into a casing 100 with a C-shaped cross-section, and light from a light-emitting unit 102 secured to an end section of the casing is introduced into the bar-shaped light guide through the end face thereof and is caused to be totally reflected within the bar-shaped light guide and emitted from a light-emitting surface of the light guide
Referring to Patent Document 1, as shown in FIG. 13, a bottom section of the bar-shaped light guide 101 is provided with a scattering pattern 103 coated with a white paint and the like and each light-emitting element 102a, 102b and 102c of three primary colors of the light-emitting unit 102 is disposed by offsetting relative to a normal line from this bottom surface.
As a structure to prevent the bar-shaped light guide from dropping out from the casing, FIG. 4 of Patent Document 2 discloses that an engaging section is provided at an end section of the casing close to the light-emitting unit. Patent Document 3 also discloses a structure whereby a casing is divided in two parts in the longitudinal direction to prevent a gap from being formed between the end section of the casing and the light-emitting unit under the influence of heat.
[Patent Document 1] Japanese Patent Application Publication No. 6-217084
[Patent Document 2] Japanese Patent Application Publication No. 2003-281913
[Patent Document 3] Japanese Patent Application Publication No. 2004-146870
In the case where the light-emitting elements of three primary colors are offset relative to the normal line from the bottom surface of the bar-shaped light guide as in Patent Document 1, the shape of the bar-shaped light guide becomes flat and as a result, the shape of the illumination device is also affected by the shape of light guide.
On the other hand, when each light-emitting element of three primary colors is disposed along the normal line from the bottom surface of the bar-shaped light guide, there are problems as described in FIGS. 14 and 15.
FIGS. 14 (a) through (c) are views describing the condition in which light from a blue-color light-emitting element 102a, a red-color light-emitting element 102b, and a green-color light-emitting element 102c are incident on the bar-shaped light guide 101, wherein light from each light-emitting element has light distribution characteristics of 180 degrees just before entering the bar-shaped light guide. However, when light is incident on a light guide made of acrylic, a radiation angle of the light from each light-emitting element has a spread of 84 degrees by Snell's law.
FIG. 15 is a graph showing the relationship between displacement and light intensity of each light-emitting element in the main scanning direction (in the longitudinal direction of the light guide). As described above, since the radiation angle of light from each light-emitting element within the bar-shaped light guide 101 is equal (about 84 degrees), the light intensity of the blue-color light-emitting element 102a which is far from the bottom surface of the bar-shaped light guide 101 does not go over 80% unless a distance more than 5 mm is kept from the incident end face of the bar-shaped light guide 101. Thus, when even one color has insufficient light intensity, that section becomes a redundant area which cannot be used as a light source.
In the conventional technology, the redundant area is long and the length of a section useless as the illumination device is great. Those problems can not be solved by the structures disclosed in Patent Documents 2 and 3.